What's the use of Ruby on rails ?

Answer 1

It enables the programmer to easily create advanced database-driven websites using scaffolding and code generation, following convention over configuration, which means that if you stick to a certain set of conventions, a lot of features will work right out of the box with very little code.

Answer 2

Rails (usualy called Ruby on Rails, or RoR) is an advanced web server Ruby framework.
Basically, it's used to handle server sided http requests by sending dynamic html pages and/or any files to user side (web browsers).
Anyway, as stated by Ekansh kashyap, as "advanced" framework it is designed to help to structure your work easiest and quickest than by writting by yourself all the needed stuff to handle database and dynamical templating through MVC paradigm...
https://www.sitepoint.com/model-view-controller-mvc-architecture-rails/